1. Technical Field of the Invention
The present invention relates generally to a gas sensor which may be installed in an exhaust system of an internal combustion engine to determine the concentration of O2, NOx, HC, or CO in exhaust emissions, and more particularly to an improved structure of such a type of gas sensor designed to ensure a quick response to a change in, for example, concentration of a gas to be measured without sacrificing the effect of avoiding breakage of a sensor element.
2. Background Art
Conventionally, gas sensors are used for burning control of internal combustion engines for automotive vehicles. As a typical example, a gas sensor is installed in an exhaust pipe of an automotive engine to measure the concentration of a specified gas contained in exhaust emissions of the engine. A gas sensor of this type consists essentially of a gas sensor element disposed within a hollow cylindrical housing, an air cover installed on a base portion of the housing, and a protective cover assembly installed on a tip portion of the housing. The protective cover assembly has a double-walled structure made up of an inner and an outer cylindrical cover. The inner cover is smaller in diameter than the outer cover and disposed inside the outer cover coaxially.
The protective cover assembly has formed in the inner and outer covers a plurality of gas inlets through which the exhaust emissions enter a gas chamber defined in the cover assembly. The sensor element measures the concentration of the specified gas such as oxygen in the exhaust emissions admitted into the gas chamber.
Accurate measurement of the concentration of the specified gas subject to change cyclically requires use of gas sensors designed to provide a quick response to such a change. Further, the gas sensor element is apt to be wetted with moisture contained in the exhaust emissions, and may be broken. In order to avoid this, it is required to minimize the quantity of water entering the gas chamber through the gas inlets of the inner and outer covers.
Increasing the response rate of the gas sensors requires increasing the size of the gas inlets of the inner and outer covers, but it results in ease of intrusion of the moisture into the gas chamber. This is objectionable in avoiding the wetting of the gas sensor.
Single-walled protective covers are also known which are designed to simplify the flow of gas into the gas chamber for improving the response rate of the gas sensor, but it causes the sensor element to get wet directly with the moisture, thus accelerating the breakage such as cracks in the sensor element.